1. Field of the Invention
The present invention generally relates to a pulse wave detecting apparatus and particularly to such an apparatus which is capable of changing the outward advance amount of a first press surface in which a pressure sensing device is provided, from a second press surface surrounding the first press surface.
2. Related Art Statement
The Assignee of the present U.S. patent application had filed a Japanese utility model application, which was published under Publication No. 2-79904 on Jun. 20, 1990, which discloses an apparatus for detecting a pressure pulse wave produced from an artery of a living subject, including (a) a first press member having a first press surface, (b) a pressure sensing device supported by the first press surface, (c) a second press member having a second press surface surrounding the first press surface, (d) a pressing device pressing the first and second press members toward a body surface of the subject so that the first press surface is pressed against the artery of the subject via the body surface, the pressure sensing device generating an electric signal representing the pressure pulse wave produced from the artery and transmitted thereto via the body surface, the first and second press members being displaceable relative to each other in a direction in which the first and second press members are pressed by the pressing device, and (e) an actuating device displacing the first press member relative to the second press member so that the first press surface is advanced outward from the second press surface.
In the case where an artery from which the above apparatus is to detect pressure pulse wave is located at a small depth from the body surface on which the apparatus is set, the actuating device holds the first press surface at a small outward advance amount from the second press surface while the pressing device presses the second press member on the body surface. Thus, the apparatus is free from the problem that because of, for example, physical motion of the subject, the first press member may be tilted with respect to the artery while the first press member is pressed on the body surface. In this respect, the apparatus stably detects the pressure pulse wave. Meanwhile, in the case where the artery is located at a large depth from the body surface, the apparatus stably detects the pressure pulse wave by holding the first press surface at a large outward advance amount from the second press surface, without having to press the second press surface so strongly as the first press surface. Accordingly, the apparatus is free from the problem that the second press surface is pressed strongly against the bone or tendon of the subject, preventing the subject from feeling discomfort due to the strong pressing.
However, the above apparatus is adapted to determine an optimum pressing force of the pressing device at which the pressure sensing device detects the pressure pulse wave, by pressing both the first and second press members against the body surface and, if the determined optimum pressing force is greater than a reference value, retracting the second press member relative to the first press member, such that these operations are repeated until a determined optimum pressing force becomes not greater than the reference value. If the determined optimum pressing force is greater than the reference value, the apparatus gives the subject some discomfort since the second press member is pressed for some time against the body surface together with the first press member at a force greater than the optimum pressing force. In addition, if the pressing and retracting operations are repeated many times, the subject feels much discomfort. Thus, the above apparatus is not satisfactory.